Participants who meet eligibility requirements will enter the single-blind Filter Period. Participants with either sitting systolic blood pressure (SiSBP) or sitting diastolic blood pressure (SiDBP) >=95th percentile for gender/age/height will be administered either losartan 25 mg or losartan 50 mg depending on body weight (<50 kg and >=50 kg, respectively). After 3 weeks, if SiSBP or SiDBP is >=95th percentile, participants will titrate to either losartan 50 mg or 100 mg according to weight. Participants with both SiSBP and SiDBP <95th percentile will be excluded from the study. After an additional 3 weeks, participants whose blood pressure (either SiSBP or SiDBP) remains >=95th percentile will then enter the double-blind phase. Participants who meet all entry criteria will be randomized in a 1:1 fashion to either losartan or losartan/hydrochlorothiazide (HCTZ) for 4 weeks and will be stratified according to body weight (<50 kg and >=50 kg). Participants whose weight is <50 kg will be randomized to losartan 50 mg or losartan50 mg/HCTZ 12.5 mg. Participants weighing >=50 kg will be randomized to losartan 100 mg or losartan/HCTZ 100 mg/12.5 mg. At the end of double-blind treatment, participants may enter a 20-week double-blind extension and will continue on their randomized therapy with losartan or losartan/HCTZ.